Hi all. I finished my ST-120 build a couple of weeks ago and all was fine with no issues. I let it burn in for maybe 10 hours or so with no issues except with some static in the speakers (not hum) as it was warming up. The bias on all output tubes was stable at .55 volts. Jump ahead a week (tonight). I turned the amp on and saw a momentary flash in the center tube and a few seconds later I heard a startling crackle from the speakers as well and a small but noticeable flash from the right rear KT88. The amp seems to play fine but when I checked the bias on the tubes the right rear KT88 won't bias properly. With the trimmer pot turned all the way counter clockwise bias on that tube is 26 volts. It was also 26 volts where the old .55 volt setting was. Am I to assume the tube is bad? Bias is fine on all others. I even switched out the center tube and although with no flash in it on start up the right rear tube still sits at 26 volts. I'm using JJ 12AU7's and Shuguang KT88's. Any suggestions would be appreciated although I'm thinking it's a bad tube. Cheers...Leigh
4 posters
VTA ST-120 Bias
svirant- Posts : 3
Join date : 2013-03-25
Check the value of the 10 ohm resistor that is beneath the KT88 tube. I had the same thing happen to me and it was a bad resistor. When mine went bad it had a value of 4 Mohms and I too was getting readings of 27-28 volts. Replaced the resistor and all was fine, including the tube.
Bob Latino- Admin
- Posts : 3276
Join date : 2008-11-26
Location : Massachusetts
Leigh,
svirant has given good advice ... Every now and then an output tube will develop a short and draw too much current. When this happens sometimes the 10 ohm bias resistor on that socket will either open or change resistance. Measure the resistance of the 10 ohm resistor on that tube socket. If it doesn't measure 10 ohms, I will send you another 10 ohm 2 watt resistor. From experience it is more likely that the tube caused the 10 ohm resistor to go bad than vice versa.
I would replace both that KT88 and the center driver tube. You did not get your tube set from me. If the vendor that you got the tubes from is honest and reliable he should replace those two tubes for you at no cost to you. The standard tube warranty is 90 days. I don't think you have had your tubes 90 days. You will have to send the tubes that are bad back to the vendor.
Bob
svirant has given good advice ... Every now and then an output tube will develop a short and draw too much current. When this happens sometimes the 10 ohm bias resistor on that socket will either open or change resistance. Measure the resistance of the 10 ohm resistor on that tube socket. If it doesn't measure 10 ohms, I will send you another 10 ohm 2 watt resistor. From experience it is more likely that the tube caused the 10 ohm resistor to go bad than vice versa.
I would replace both that KT88 and the center driver tube. You did not get your tube set from me. If the vendor that you got the tubes from is honest and reliable he should replace those two tubes for you at no cost to you. The standard tube warranty is 90 days. I don't think you have had your tubes 90 days. You will have to send the tubes that are bad back to the vendor.
Bob
Leigh W- Posts : 47
Join date : 2013-04-02
Thanks very much for the info Bob. I opened the amp up and sure enough the 10 ohm resister was burnt (infinite) resistance. Fortunately for me the Tube store is only 20 minutes away so I took the KT88 and 12au7 back and they gave me two new ones. He put my originals on his tube tester and he said both tubes checked out perfectly but that the tube tester doesn't always tell the whole story. Thanks for the offer on mailing me out a new resister but I picked a couple up at our local electronics store so all is good. I will replace the resister this afternoon and post on the board as to the outcome. While I have the amp on the bench should I be looking at anything else that may have caused the problem or are you pretty sure the tubes were to blame? Thanks again...Cheers!
Leigh W- Posts : 47
Join date : 2013-04-02
And thanks as well to svirant for starting me off in the right direction.
Bob Latino- Admin
- Posts : 3276
Join date : 2008-11-26
Location : Massachusetts
Hi Leigh,
That 10 ohm 2 watt resistor will only burn out IF the that tube draws too much current. The tube will only draw too much current if it has some type of internal short. In every instance that I know of, the defective tube took out the resistor - not vice versa.
You should be OK with the new tubes and another 10 ohm 2 watt resistor. Note - that 10 ohm resistor is that tube's only reference to chassis ground. Without that resistor in there, that tube basically "won't work" even though the tube's filament may still glow. That channel may even continue to play but that channel is only getting the benefit from other output tube of the pair.
Bob
That 10 ohm 2 watt resistor will only burn out IF the that tube draws too much current. The tube will only draw too much current if it has some type of internal short. In every instance that I know of, the defective tube took out the resistor - not vice versa.
You should be OK with the new tubes and another 10 ohm 2 watt resistor. Note - that 10 ohm resistor is that tube's only reference to chassis ground. Without that resistor in there, that tube basically "won't work" even though the tube's filament may still glow. That channel may even continue to play but that channel is only getting the benefit from other output tube of the pair.
Bob
Leigh W- Posts : 47
Join date : 2013-04-02
Great Bob. That puts my mind at ease. Will let you know how I make out. Leigh
Roy Mottram- Admin
- Posts : 1838
Join date : 2008-11-30
BTW, a 2 watt 10 ohm resistor would need more than 4-5 volts across it to burn out.
2 watts is nearly 80X the normal power going thru that resistor.
You could even use a 1/4w resistor and still have 5X as much power as needed.
That cheap 10 ohm resistor is a really cheap fuse for a bad tube.
2 watts is nearly 80X the normal power going thru that resistor.
You could even use a 1/4w resistor and still have 5X as much power as needed.
That cheap 10 ohm resistor is a really cheap fuse for a bad tube.
Leigh W- Posts : 47
Join date : 2013-04-02
Hi Bob. Replaced the 10 ohm resister, KT88 and 12AU7 and it works perfectly. I remember noticing that when I first powered it up after the build that as the tubes were warming there was a concerning amount of static from the right speaker until the tubes warmed up. I thought maybe this was a normal occurrence. Not now. Dead silence on warm up. Seems to me on looking back now that maybe this KT88 may have been "iffy" right from the start. Anyway, I'm listening to some smooth jazz in triode mode and enjoying every minute. She looks pretty darn good in the dark too. Love the way the KT88's glow. Thanks again for your help and input. Cheers! Leigh
svirant- Posts : 3
Join date : 2013-03-25
Leigh - Glad to hear it all worked out for you! 
Leigh W- Posts : 47
Join date : 2013-04-02
Thanks so much for starting me out on the right path. It's playing beautifully right now. Just listening to some old vinyl and reminiscening. Cheers!...Leigh